Mercruiser 5.0 Won’t Reach Operating Temp

[dremison]

I have a 2007 Mercury engine that I just replaced the thermostat in since it wouldn’t reach operating temperature. It reached 160 just fine in the driveway but won’t get past 120 under load on the water still. What else could be causing this issue?

 

[alldodge]

The motor doesn't get as much water on a hose as it does in the water
Motor Serial number please?

 

[dremison]

Motor Serial Number OM663009. Thanks for the reply!

 

[alldodge]

Seeing your cooling system is a 7 point drain.
Was the thermostat and all the pieces installed as shown? Note the thermostat goes against housing, then the gasket, then sleeve

Also was the housing clean of any left over gasket material?

 

[dremison]

Confirmed. It wasn’t until recently that the engine wouldn’t get up to temp and replacing the thermostat did not fix it while under load.

 

[alldodge]

Water goes into the block from the circulating water pump and what keeps it from coming back out is the thermostat. The Stat is just a valve. If the Stat doesn't open it over heats, if it gets stuck open then it runs cool. Since its not the Stat then only other thing is the housing.

Either that or it is getting warm enough its just the gauge. Have you tried and checked temp with a IR temp gun at the sender location?

 

[Scott06]

Confirmed. It wasn’t until recently that the engine wouldn’t get up to temp and replacing the thermostat did not fix it while under load.

As a sanity check - Have you verified the 120 is indeed accurate with an IR thermometer also the OEM merc thermostat gasket has rivets in it to ensure the housing sensor is properly grounded to manifold. Can cause issues if a aftermarket gasket with out rivets is used

I think you can check the gauge by taking the wire off the sender and grounding it, gauge should peg high

 

[dremison]

As a sanity check - Have you verified the 120 is indeed accurate with an IR thermometer also the OEM merc thermostat gasket has rivets in it to ensure the housing sensor is properly grounded to manifold. Can cause issues if a aftermarket gasket with out rivets is used

I think you can check the gauge by taking the wire off the sender and grounding it, gauge should peg high

I will do both of those and get back to you. I’m also going to make sure I didn’t mess up the gasket on the thermostat placement. Thank you in advance.

 

[QBhoy]

Check the actual temp for sure. My v8 mpi dash gauge shows about 10-15o difference from the actual reading from smartcraft ecu. Sits steady at 152 usually

 

[Fun Times]

I’m also going to make sure I didn’t mess up the gasket on the thermostat placement.

Unfortunately in this case the image location of the smaller cork type gasket is a bit misleading vs actual position… The cork gasket goes in the housing first then the thermostat all like seen in the following video,

While in there check to ensure there’s no newer type debris holding the thermostat open since installing the new thermostat.

Also just in case you bought a “kit” it shouldn’t be an O-ring in there.

 

[tank1949]

If my old memory serves me correctly, the thermostat gasket is a special one that has grounding "tits" that must touch intake manifold.

 

[dremison]

Unfortunately in this case the image location of the smaller cork type gasket is a bit misleading vs actual position… The cork gasket goes in the housing first then the thermostat all like seen in the following video,

While in there check to ensure there’s no newer type debris holding the thermostat open since installing the new thermostat.

Also just in case you bought a “kit” it shouldn’t be an O-ring in there.

I did put a layer of permatex on the bottom between the manifold and the first gasket - I wonder if that is affecting the communication. I’ll take it apart tomorrow and investigate.

 

[dremison]

I did put a layer of permatex on the bottom between the manifold and the first gasket - I wonder if that is affecting the communication. I’ll take it apart tomorrow and investigate.

That’s also the exact video I pulled when I started this project, too. Great minds…

 

[dremison]

Water goes into the block from the circulating water pump and what keeps it from coming back out is the thermostat. The Stat is just a valve. If the Stat doesn't open it over heats, if it gets stuck open then it runs cool. Since its not the Stat then only other thing is the housing.

Either that or it is getting warm enough its just the gauge. Have you tried and checked temp with a IR temp gun at the sender location?

Update: replaced the new thermostat and I did have the top gasket in the wrong spot. Got up to temperature under load and stayed there.
However, it started to overheat when I got it home and used a hose - never done that before. Is that normal?

 

[alldodge]

Update: replaced the new thermostat and I did have the top gasket in the wrong spot. Got up to temperature under load and stayed there.
However, it started to overheat when I got it home and used a hose - never done that before. Is that normal?

Its not normal, might need to replace the impeller
Have you verified the temp on the gauge with a IR?

 

[dremison]

I did verify. Why would it overheat on land but not in the water? All I did was change out the thermostat and move a gasket but now the impeller is bad? How would that not have been picked up before when I had a different thermostat that was stuck open?

 

[alldodge]

Don't know if impeller is bad, but it was running to temp on a hose and the Stat was installed incorrectly. Normally even if the impeller is bad or week a hose will push enough water thru to keep the motor within limits, by pushing the water thru. You also have the spring and check balls which means if they get stuck open from something getting caught, then more water would go to the exhaust elbow instead of to the manifold. The balls are used mainly when the boat is on planing speed to allow excess water to go straight out

In the water the impeller can move more water because (assuming this is a Alpha drive) the water is pushed thru from the drive intake holes which are made to force water into them while moving. If the impeller is completely gone it would overheat in any case

Could also be there is very low water pressure on the hose, but this was not an issue prior so "assume" this is not an issue now.

 

[Fun Times]

When you run it on the garden hose at home, is there any water coming out or the exhaust ports at the side of the transom assembly? And or out the center of the prop sometimes more when reved up?
What RPM are you running the engine at on the trailer? Should be 1300 rpm.
Make sure the ear muffs are on covering the holes really good tight and centered with water all the way turned up if you have low water psi at the house… if high psi, then be careful the muffs don’t slide off the holes area…if you have any air in the system it will overheat.

 

[dremison]

Don't know if impeller is bad, but it was running to temp on a hose and the Stat was installed incorrectly. Normally even if the impeller is bad or week a hose will push enough water thru to keep the motor within limits, by pushing the water thru. You also have the spring and check balls which means if they get stuck open from something getting caught, then more water would go to the exhaust elbow instead of to the manifold. The balls are used mainly when the boat is on planing speed to allow excess water to go straight out

In the water the impeller can move more water because (assuming this is a Alpha drive) the water is pushed thru from the drive intake holes which are made to force water into them while moving. If the impeller is completely gone it would overheat in any case

Could also be there is very low water pressure on the hose, but this was not an issue prior so "assume" this is not an issue now.

What do you recommend at this point?

 

[dremison]

When you run it on the garden hose at home, is there any water coming out or the exhaust ports at the side of the transom assembly? And or out the center of the prop sometimes more when reved up?
What RPM are you running the engine at on the trailer? Should be 1300 rpm.
Make sure the ear muffs are on covering the holes really good tight and centered with water all the way turned up if you have low water psi at the house… if high psi, then be careful the muffs don’t slide off the holes area…if you have any air in the system it will overheat.

I don’t run it w the RPMs at 1300. Just at idle before and never had any overheating issues. I also have a top down water system and run the hose through there.